The industrial processes for the production of lead batteries conventionally envisage a step for assembly of the individual voltaic cells which make up the batteries, with the provision, inside plastic containers, of lead plates or grids containing the active material (lead, lead sulfate, etc) which is involved in the electronic chemical reactions. The positive and negative plates are connected electrically together to form the electrodes, are immersed in a concentrated aqueous solution of acid, referred to below as electrolytic solution, generally consisting of sulfuric acid (electrolyte), and are electrically divided by partitions for isolating the electrodes from each other, while allowing free circulation of the electrolyte.
The plates are therefore those elements where, during discharging and charging of the batteries, electrochemical reactions occur for transformation of chemical energy into electrical energy and vice versa. During discharging the anode is subject to an oxidizing reaction and the cathode to a reducing reaction which enable an EMF to be generated at the electrodes.
At the time of their production, the plates are inert or inactive and are made active by means of an electrochemical formation process for transforming the oxide and the lead sulfate into spongy metallic lead and into lead dioxide which form the active material of the respectively negative and positive plates.
This operation for electrical formation of the plates requires the supplying of direct current to the electrodes of the cells forming the batteries so that they are charged up to the predefined voltage and current values and are ready for commercial distribution and use.
During formation of the plates there is an increase in the temperature of the electrodes and the electrolytic solution both owing to the endothermic chemical reactions which take place during charging and as a result of the ohmic effect due to the flow of the direct current.
A problem which exists in the industrial battery-manufacturing sector consists in the need to accelerate the formation process without, however, increasing the charging current to excessively high values which could damage the active material.
For this purpose, various formation methods and plants have been developed in order to control the temperature during the process for electrochemical formation of the plates, with the aim of preventing damage to the active material which lines them. In order to achieve this, the temperature must preferably be kept below 60° C.
In accordance with a known plant for the electrical formation of lead-acid batteries, in order to cool the electrolytic solution during charging, said solution is extracted from the battery cells and is circulated inside an external circuit where its temperature characteristics and electrolytic concentration are controlled.
It has also been attempted to solve the problem of heating by circulating inside the battery cells an electrolytic solution which is diluted or has a low electrolytic density so as to produce a small amount of heat during the main battery formation stage and an electrolytic solution with a higher density for final charging of the batteries with an electrolyte having a final concentration substantially equal to the concentration for operation of the battery.
For this purpose, the external circuit for circulating the electrolytic solution comprises two circuits, each of which has, connected along it, a storage tank for the respective electrolytic solution, a heat exchanger and means for adjusting the density of the solution by means of the controlled supply of water or electrolyte.
The sulfuric acid concentration must remain substantially constant during charging. However, the chemical reactions produce a variation in the concentration of the electrolyte. In particular, during the formation stage there is an increase in the concentration of the electrolytic solution and consequently the density adjusting means introduce preset amounts of solvent liquid (usually water).
Normally the electrolytic solution is introduced with force inside each cell so that the electrolyte already present is forced out under pressure.
For this purpose, as is known, each voltaic element (or cell), referred to below in short as “cell”, is provided with a closing cap inserted inside an opening provided on top of the lid of its plastic container.
Each cap usually comprises an inlet pipe, through which the electrolytic solution is introduced into the cell, and an outlet pipe, through which the electrolytic solution is extracted from the cell. The inlet and outlet pipes in the cap of each cell are respectively connected by means of first and second connecting lines to an electrolytic solution distribution header, which receives the solution from a delivery line, and to an electrolytic solution receiving header, which conveys the solution to a return line. This circulation circuit, via the pipes in the cap, ensures that there is a continuous flow of electrolyte passing through the individual cells.
In greater detail, the end of the inlet pipe inside the cell extends down to a height lower than that which is reached by the inner end of the outlet pipe.
The inlet pipe has a diameter with dimensions specifically designed to supply the cell with a predetermined flow usually determined by the piezometric height at which the distribution header is situated. Heating of the cells and the pressure acting on them in order to circulate the electrolytic solution may result in deformation of the cell containers.
Moreover, during the formation stage, the electrochemical reactions which occur inside the cells may generate inflammable gases and in particular hydrogen. The latter may accumulate in the top part of the cell underneath its lid, without being able to escape through the outlet pipe. Moreover, the hydrogen atmosphere underneath the lid may be enriched also with oxygen which has formed during separation of the water in particular during the final charging stage when the plates are less likely to absorb the sulfates.
Consequently, dangerous explosions may occur in the event of short-circuits between plates of different polarity.
In order to overcome these drawbacks, it is known, for example, from EP-A-1627438 to extract the electrolytic solution from the cells by providing a vacuum using suitable fans in the electrolytic solution return line and providing an opening in the cap suction connection pipes in order to dilute the quantity of hydrogen, or inflammable gases, which are conveyed along the return line. In greater detail, in accordance with the teaching expressed in this patent on Page 11, lines 3 to 6, the gas expelled by the fan contains a quantity of hydrogen below the limit which triggers explosion thereof in air, because it is diluted in each of the pipes connected to the return pipe, said pipes each having an opening which allows the entry of sufficient volumes of air.
This solution, however, in practice has proved to be not without drawbacks. In fact, most of the accidents due to explosion of the hydrogen/oxygen combustion mixture occur in the proximity of the battery and not along the circuit for drawing off the electrolytic solution.
As is known, in fact, the risks of explosion are mostly due to small discharges which occur not so much when the electrolytic solution is drawn off and conveyed, but more so inside the battery and, even more frequently, on the outside thereof.
As is known, the plates with different polarity are alternated with each other inside the battery and are connected to two ordinary electrodes which are welded to the lead sleeves fixed to the lid. A voltage differential of 2 volts exists between the plates of different polarity and may give rise to discharges, for example due to faults or holes in the dividing partitions, causing triggering of the explosive mixture.
Before charging, the electrodes of the various cells are connected to electrical connectors so that the batteries are arranged in series and connected to the power supply source. The batteries are often damp with the sulfuric acid solution which, attacking the lead of the electrodes, forms lead sulfate which is an insulating agent. When the operator moves the connectors in order to re-establish electrical conduction, electrical discharges may occur between the connector and the electrode outside the battery with consequent triggering of the explosive combustion mixture.
In the sector for the manufacture of industrial batteries, in particular large-size batteries, such as in particular static batteries, filling of the cell container with the electrolyte is a fairly lengthy process because of the large quantity of electrolytic solution which must be introduced into the containers before they are filled, so as to allow the electrolytic solution to start circulating inside the external circuit and to cool as a result.
Operationally speaking, in order to perform electrolytic formation, the batteries to be charged are arranged on pallets and are then electrically connected to an electric power source (mains power supply) and then the caps are hydraulically connected to both the delivery and return connecting lines so as to then start the plant charging process.
The inlet pipe in each cap is, however, specifically designed for the cell operating condition so that initially a lot of time is required to fill the container of each cell with the electrolytic solution before the latter reaches the outlet pipe of the cap and can start to circulate inside the external cooling circuit.
In addition to the drawback of the long time required for electrochemical formation of the plates (or charging of the batteries) owing to the time lost with filling the cells, the chemical reactions which occur at the start of filling may cause excessive heating of the electrolytic solution, which risks damaging the active material.
For these reasons, the large-size batteries are usually filled manually before being charged, by inserting the free ends of pipes connected to a storage tank containing the electrolytic solution, into the openings in the caps of the cells, before closing them with the caps.
This procedure is awkward and lengthy to perform and requires personnel to carry out filling of all the cell containers.
Moreover, in the event of a malfunction of the suction system intended to provide a vacuum in the return line, the electrolytic solution flows out from the end of the compensation pipe into the air and is dispersed, soiling the premises and the plant.